DEFINITION / CLINICAL DETERMINATION

One of the greatest challenges for nephrologists caring for haemodialysis patients is assessing their extracellular fluid volume (Ishibe & Peixto, 2004); most nephrologists have used the concept of ‘dry weight’ to drive such decisions.

‘Dry weight’ is a term which generates considerable confusion. It has been traditionally defined as “the weight at the end of a dialysis treatment below which the patient often develops symptoms of hypotension”. Although the concept of dry weight is used every day by nephrologists to prescribe an ultrafiltration goal for individual dialysis treatments, in practice this is based on trial and error rather than on hard evidence.

This complex investigation may result in the successful determination of the patients dry weight.

Consequences of insufficient volume controlThe inexact determination of dry weight results in substantial errors such that patients leave the dialysis centre (after treatment) in hypervolemic or hypovolemic states. Hypervolemic patients are at risk for developing volume-related hypertension, left ventricular hypertrophy (LVH) and congestive heart failure. Conversely, hypovolemic patients may suffer from symptoms of volume depletion and interdialytic hypotension. More accurate and rational methods of measuring the fluid overload and body composition are thus required in order to better determine a patient’s dry weight.

Clinical TerminologyThe purpose of this section is to discuss the subtle differences between clinical terms that are often used to describe a patient’s weight and fluid status.

Different weight terms used in routine practice.

Pre-dialysis weight
The pre dialysis weight (75 kg in this example) is unambiguous and obviously provides the starting point for treatment.

Normohydration weight
The normohydration weight represents the weight that would be achieved when the hydration of the patient reflects the normal hydration of body tissues that would be encountered in healthy controls with matched body composition. It is the weight obtained when zero excess fluid is present. The normohydration weight is a definition based on tissue hydration properties known from scientific analysis. It is synonymous with the clinical ideas of normovolaemia or euvolaemia or the concept of normal extracellular volume. It is essential to appreciate that although normohydration weight represents a possible target weight for a patient it does not necessarily mean that this weight can be achieved. Comorbidity and the use of antihypertensive agents are factors that influence the vascular stability of a patient during treatment.

Tolerated weight
Relative high ultrafiltration rates may lead to recurrent intradialytic symptoms. Hence, the patient may not reach the normohydration weight but the tolerated weight. In order to reach the individual dry weight of the patient, prolonged treatments and/or low ultrafiltration rates may be necessary.

Post dialysis weight
Is the weight of the patient after treatment.
It may be the normohydration weight, the tolerated weight, or clinical dry weight.